1. Field of the Invention
The present invention relates to a method and system for improving resolution of images in magnetic resonance imaging (MRI).
2. Description of the Related Art
Magnetic resonance (MR) is a phenomenon exhibited by atomic nuclei which are placed in a static magnetic field and have a nonzero magnetic moment whereby the atomic nuclei absorb and emit electro-magnetic energy only at specific frequencies through their resonance. The atomic nuclei resonate at an angular frequency .omega.o (.omega.o=2.pi..nu., .nu.=Larmor frequency) given by EQU .omega.o=.gamma.Ho (1)
where .gamma. is the gyromagnetic ratio inherent in each type of nucleus and Ho is the strength of the applied static magnetic field.
MRI apparatus utilizing the magnetic resonance phenomenon have been in wide use for medical diagnosis. An MRI apparatus detects MR signals after the resonance absorption and processes the MR signals to obtain diagnostic information, for example, slice images of a subject resulting from the density of nuclei, longitudinal relaxation time, transverse relaxation time, flow and chemical shifts, noninvasively.
The diagnostic information may be acquired by exciting the whole body of the subject placed in the static magnetic field. In practice, however, the diagnostic information is obtained by exciting only a specific portion of the subject because of limitations on the arrangement of the MRI apparatus and clinical demands for acquired images.
A high resolution version of MR images will now be discussed. The MR signals observed by the MRI correspond to points on the K space of a subject (see FIG. 1A), namely, on a frequency region (Fourier plane) having a readout direction and a phase encoding direction as shown in FIG. 1B. In FIG. 1B, the end of the frequency region corresponds to high frequency components in the subject. Hence, to estimate the original subject at high resolution, in other words, to improve resolution of reconstructed MR images, the MRI device must acquire MR data over a wide frequency range. For example, as shown in FIG. 2, images reconstructed from MR data within a narrow frequency range (hatching portion) blur.
The position of data in the K space is proportional to an integral amount of the strength of a gradient magnetic field applied to spins in the subject. To observe the high frequency components of the original data, a large integral amount of gradient magnetic field is needed. To increase the integral amount of the gradient magnetic field, the strength or application time period of the gradient magnetic field must be increased. When the application time period of the gradient magnetic field is increased, the strength of MR signals decrease because of the effect of the transverse relaxation time. Further, echo time is an important parameter relating to induction of the MR signals and thus increase of the application time is limited. For this reason, the strength of the gradient magnetic field is increased in order to increase the integral amount of the gradient magnetic field. As a result, the high frequency components of the original data are observed, and it is possible to realize a high resolution version of the MR images. However, this leads to the necessity of a large capacity power supply source for generating the gradient magnetic field. This power supply source is not a practical apparatus from the standpoint of the arrangement of the MRI apparatus.
An MRI apparatus is according desired which can improve resolution of images without the need of a large-capacity power supply source.